Action of an irreversible acetylcholine esterase inhibitor, soman, on muscarinic hyperpolarization in cat bladder parasympathetic ganglia

Age-Related Susceptibility to Epileptogenesis and Neuronal Loss in Male Fischer Rats Exposed to Soman and Treated With Medical Countermeasures

Elderly individuals compose a large percentage of the world population; however, few studies have addressed the efficacy of current medical countermeasures (MCMs) against the effects of chemical warfare nerve agent exposure in aged populations. We evaluated the efficacy of the anticonvulsant diazepam in an old adult rat model of soman (GD) poisoning and compared the toxic effects to those observed in young adult rats when anticonvulsant treatment is delayed. After determining their respective median lethal dose (LD50) of GD, we exposed young adult and old adult rats to an equitoxic 1.2 LD50 dose of GD followed by treatment with atropine sulfate and the oxime HI-6 at 1 min after exposure, and diazepam at 30 min after seizure onset. Old adult rats that presented with status epilepticus were more susceptible to developing spontaneous recurrent seizures (SRSs). Neuropathological analysis revealed that in rats of both age groups that developed SRS, there was a significant reduction in the density of mature neurons in the piriform cortex, thalamus, and amygdala, with more pronounced neuronal loss in the thalamus of old adult rats compared with young adult rats. Furthermore, old adult rats displayed a reduced density of cells expressing glutamic acid decarboxylase 67, a marker of GABAergic interneurons, in the basolateral amygdala and piriform cortex, and a reduction of astrocyte activation in the piriform cortex. Our observations demonstrate the reduced effectiveness of current MCM in an old adult animal model of GD exposure and strongly suggest the need for countermeasures that are more tailored to the vulnerabilities of an aging population.

Neurophysiologic effects of chemical agent hydrolysis products on cortical neurons in vitro

The neurophysiologic effects of chemical agent hydrolysis products were examined on cultured cortical neurons using multielectrode array (MEA) recording and the whole-cell patch clamp technique. Measurement of neuronal network extracellular potentials showed that the primary hydrolysis product of soman, pinacolyl methylphosphonic acid (PMPA), inhibited network mean burst and spike rates with an EC50 of approximately 2 mM. In contrast, the degradation product of sarin, isopropyl methylphosphonic acid (IMPA), and the final common hydrolysis product of both soman and sarin, methylphosphonic acid (MPA), failed to affect neuronal network behavior at concentrations reaching 5 mM. Closer examination of the effects of PMPA (2 mM) on discriminated extracellular units revealed that mean spike amplitude was slightly diminished to 95 +/- 1% (mean +/- S.E.M., n = 6, P < 0.01) of control. Whole-cell patch clamp records under current clamp mode also showed a PMPA-induced depression of the firing rate of spontaneous action potentials (APs) to 36 +/- 6% (n = 5, P < 0.001) of control. In addition, a minor depression with exposure to PMPA was observed in spontaneous and evoked AP amplitude to 93 +/- 3% (n = 5, P < 0.05) of control with no change in either the baseline membrane potential or input resistance. Preliminary voltage clamp recordings indicated a reduction in the occurrence of spontaneous inward currents with application of PMPA. These findings suggest that PMPA, unlike MPA or IMPA, may more readily interfere with one or more aspects of excitatory synaptic transmission. Furthermore, the data demonstrate that the combination of extracellular microelectrode array and patch clamp recording techniques facilitates analysis of compounds with neuropharmacologic effects.

Comparison of atropine pre- and post-treatment in ganglion neurons exposed to soman

The prevention and reversal of the effects of soman (pinacoloxymethylphosporyl fluoride) have been examined on the electrical properties of sympathetic ganglion neurons in vitro from the adult bullfrog Rana catesbeiana. Atropine (10 microM) pre-treatment (before soman) blocked the soman-induced decrease in the membrane potential, membrane resistance, and afterhyperpolarization (AHP) duration. Atropine post-treatment (after soman exposure) restored the soman-induced decrease in the membrane potential but was ineffective in reversing either the membrane resistance or the duration of the AHP. These results demonstrate: 1) that the effects of soman on the electrical properties of these neurons are mediated by the activation of muscarinic receptors, 2) that following receptor activation different cellular mechanisms may be involved in the regulation of the electrical properties of the neuron, and 3) pre-treatment rather than post-treatment with atropine is more effective in blocking these direct effects of soman. These results are discussed in reference to the increased effectiveness of atropine pretreatment in the protocols for soman-induced neurotoxicity.

Vx enhances neuronal excitability and alters membrane properties of Rana catesbeiana sympathetic ganglion neurons

1. The effects of VX (10 microM) were examined on sympathetic ganglion neurons from the bullfrog using intracellular recording techniques. 2. VX significantly increased the amplitude of the residual EPSP from 4.8 +/- 0.86 mV (n = 4) to 13.7 +/- 1.23 mV (n = 4). 3. VX significantly decreased the membrane potential 5.2 +/- 0.75 mV (n = 6). The input resistance and the duration of the spike afterhyperpolarization (AHP) were also reduced 69.8% and 69.6% of control, respectively. 4. VX increased neuronal excitability greater than 200% (n = 5) of control. 5. The VX-induced neuronal excitability may result from a reduction in the duration of the AHP and contribute to the CNS toxicity.

Evidence for inhibitory nicotinic and facilitatory muscarinic receptors in cholinergic nerve terminals of the rat urinary bladder

Cholinergic prejunctional modulatory receptors on parasympathetic nerves in the rat urinary bladder were studied by measuring 3H-acetylcholine (ACh) release in muscle strips from the bladder body. Electrical field stimulation markedly increased 3H-ACh overflow in strips preloaded with 3H-choline. Oxotremorine (1 microM), an M2 receptor agonist and DMPP (10 microM) a nicotinic (N) receptor agonist decreased the release of ACh (50% and 55% respectively); whereas McN-A 343 (50 microM) an M1 receptor agonist increased the release (33%), indicating the presence of three types of modulatory receptors. The anticholinesterase agent, physostigmine in concentrations of 1, 5 and 25 microM and neostigmine (5 microM) increased ACh release (44-710%). However a low concentration of physostigmine (0.05 microM) decreased release. Pirenzepine, an M1 muscarinic antagonist or atropine blocked the increased ACh release in physostigmine-treated strips, but in normal strips pirenzepine did not change release and atropine increased release. McN-A 343 or prolonged application (15 min) of DMPP increased ACh release (376% and 391% respectively) in physostigmine-treated strips. The response to McN-A 343 was blocked by pirenzepine. d-Tubocurarine (DTC), a nicotinic receptor blocker, enhanced ACh release in the presence of physostigmine but proved to be ineffective in normal preparations. These findings suggest that all three cholinergic receptors (M1 facilitatory, N inhibitory and M2 inhibitory) are activated by endogenous ACh in physostigmine treated preparations whereas only M2-inhibitory receptors are activated in normal preparations. It will be important in future studies to determine whether M1 and M2 mechanisms can also be activated under more physiological conditions in the bladder and whether they are present at other cholinergic synapses.

The effects of soman on the electrical properties and excitability of bullfrog sympathetic ganglion neurones

1. The effects of soman (0.1-10 microM), an irreversible inhibitor of acetylcholinesterase (AChE), were examined on the electrical properties of ganglion neurones of the paravertebral sympathetic chain of the bullfrog, Rana catesbeiana. 2. Soman (10 microM) depolarized 29 of 35 (83%) ganglion neurones studied by 6.4 +/- 0.65 mV within 10 min of application and reduced the cell input resistance in 9 of 11 neurones examined (82%) to 55 +/- 5.3% of control. 3. Soman (10 microM) significantly reduced the maximum amplitude and the maximum rate of rise of the action potential and the duration, but not the amplitude, of the after-hyperpolarization (AHP) following the action potential elicited by either direct or antidromic stimulation. The maximum rate of fall and the duration of the action potential were not significantly affected by soman. These actions of soman were independent of the agent-induced depolarization. When examined by a single microelectrode voltage clamp, soman reduced the amplitude and the time constant of the current underlying the slow AHP, IAHs. 4. Soman (1-10 microM) produced an increase in neuronal excitability which was evidenced as either an increase in the number of action potentials or a decrease in the interspike interval in response to constant-current depolarizing pulses. The soman-induced increase in excitability occurred independently of both the agent-induced depolarization and the decrease in input resistance, was reversible with washing, was not caused by an inhibition of the M-current and was also recorded in dissociated sympathetic ganglion neurones.5. The effects of soman on the membrane potential, input resistance and the duration of the AHP but not cell excitability were blocked by pretreatment with atropine (10 microM). Pretreatment with dihydro-/J-erythroidine (DHbetalE) (10 microM) was ineffective in blocking or reversing the effects of soman. These results suggest that the direct actions of soman on the electrical properties of these neurones are mediated by activation of muscarinic receptors.